U.S. patent number 6,946,040 [Application Number 10/244,524] was granted by the patent office on 2005-09-20 for shape memory alloy and method of treating the same.
This patent grant is currently assigned to Toki Corporation Kabushiki Kaisha. Invention is credited to Dai Homma.
United States Patent |
6,946,040 |
Homma |
September 20, 2005 |
Shape memory alloy and method of treating the same
Abstract
A method of treating a shape memory alloy to improve its various
characteristics and to cause it to exhibit a two-way shape memory
effect. A raw shape memory alloy having a substantially uniformly
fine-grained crystal structure is prepared and then its crystal
orientations are arranged substantially in a direction suitable for
an expected operational direction, such as tensile or twisting
direction or the like, in which the shape memory alloy is expected
to move when used in an actuator after the completion of the
treatment.
Inventors: |
Homma; Dai (Yokohama,
JP) |
Assignee: |
Toki Corporation Kabushiki
Kaisha (Tokyo, JP)
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Family
ID: |
18702104 |
Appl.
No.: |
10/244,524 |
Filed: |
September 17, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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871619 |
Jun 4, 2001 |
6596102 |
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Foreign Application Priority Data
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Jul 6, 2000 [JP] |
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2000-204927 |
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Current U.S.
Class: |
148/563;
148/402 |
Current CPC
Class: |
C22F
1/006 (20130101); C22F 1/10 (20130101) |
Current International
Class: |
C22F
1/00 (20060101); C22F 1/10 (20060101); C22F
001/10 (); C22K 001/00 () |
Field of
Search: |
;148/402,563 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 460 695 |
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Dec 1991 |
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EP |
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59-38367 |
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Mar 1984 |
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JP |
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3-229844 |
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Oct 1991 |
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JP |
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7-76747 |
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Mar 1995 |
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JP |
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Other References
Ishikawa et al, (Shape Memory Treatment) (P. 30) 1987 (English
Translation). .
Yuuichi Suzuki, "Practical Shape Memory Alloy" published by
Kougoyuchousakai pp. 76-79, with English translation. .
"Shape Memory Alloy", by Kazuhiro Ohtuka et al., published by
Sangyoutosho pp. 157-161, with English translation..
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Primary Examiner: Wyszomierski; George
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch, LLP
Parent Case Text
This application is a divisional of application Ser. No.
09/871,619, filed on Jun. 4, 2001, now U.S. Pat. No. 6,596,102, the
entire contents of which are hereby incorporated by reference and
for which priority is claimed under 35 U.S.C. .sctn. 120; and this
application claims priority of application Ser. No. 2000-204927
filed in Japan on Jul. 6, 2000 under 35 U.S.C. .sctn. 119.
Claims
What is claimed is:
1. A shape memory alloy being polycrystallane and having a
substantially uniformly fine-grained crystal structure, crystal
orientations thereof being arranged substantially along a direction
suitable for an expected operational direction wherein said shape
memory alloy is selected from the group consisting of a wire having
a solid round cross section, a plate, and a coil, and wherein said
shape memory alloy is prepared by a process comprising the steps
of: (a) providing a raw shape memory alloy having a substantially
uniformly fine-grained crystal structure; and (b) arranging crystal
orientations of said raw shape memory alloy substantially along a
direction suitable for an expected operational direction wherein
step (a) comprises the step of: (c) heating said raw shape memory
alloy in an amorphous state or a state similar thereto to the
temperature at which recrystallization begins or a little above for
a short period of time, with a stress applied to said raw shape
memory alloy in said expected operational direction at least in the
stage where a recovery recrystallization begins, to produce a
substantially uniform fine-grained crystal structure with an
anisotropy in said expected operational direction, while relaxing
the internal stress generated in said raw shape memory alloy in
said expected operational direction; and step (b) comprises the
steps of: (d) subjecting said raw shape memory alloy to a high
level of deformation by means of a stress in said expected
operational direction at a very low temperature at which the
austenite phase does not remain in said raw shape memory alloy so
that a slide deformation is introduced into the crystal grains of
said raw shape memory alloy which have been transformed completely
into the martensite phase within a reversible range along the
direction of said stress; (e) heating said raw shape memory alloy
to a temperature between A.sub.f and the recrystallization
temperature with a stress applied to said raw shape memory alloy in
said expected operational direction so that the directions of
reversible slip motions of the respective crystal grains of said
raw shape memory alloy are arranged in a direction suitable for
said expected operational direction.
2. A shape memory alloy as set forth in claim 1, wherein the
average grain diameter of crystals is 10 microns or less.
3. A shape memory alloy as set forth in claim 1, wherein prior to
step (c), said raw shape memory alloy is subject to a severe cold
working so that the crystal structure thereof is destructed and is
brought to a state similar to an amorphous state.
4. A shape memory alloy as set forth in claim 1, wherein said shape
memory alloy is an intermetallic compound.
5. A shape memory alloy as set forth in claim 4, wherein said shape
memory alloy is a Ti--Ni based alloy.
6. A shape memory alloy as set forth in claim 4, wherein said shape
memory alloy is a Ti--Ni--Cu based alloy.
7. A shape memory alloy as set forth in claim 1, wherein said
expected operational direction is a tensile direction.
8. A shape memory alloy as set forth in claim 1, wherein said
expected operational direction is a torsion direction.
9. A shape memory alloy as set forth in claim 1, wherein said shape
memory alloy is in a form of a wire.
10. A shape memory alloy being polycrystalline and having a
substantially uniformly fine-grained crystal structure, crystal
orientations thereof being arranged substantially along a direction
suitable for an expected operational direction wherein said shape
memory alloy is selected from the group consisting of a wire having
a solid round cross section, a plate, and a coil, and
wherein said shape memory alloy is prepared by a process comprising
the steps of: (g) subjecting a raw shape memory alloy having an
anisotropy in an expected operational direction to a high level of
deformation by means of a stress in said expected operational
direction at a very low temperature at which the austenite phase
does not remain in said raw shape memory alloy so that a slide
deformation is introduced into the crystal grains of said raw shape
memory alloy which have been transformed completely into the
martensite phase within a reversible range along the direction of
said stress; (h) heating said raw shape memory alloy to a
temperature between the austenite transformation terminate
temperature A.sub.f and the recrystallization temperature with a
stress applied to said raw shape memory alloy in said expected
operational direction so that the directions of reversible slip
motions of the respective crystal grains of said raw shape memory
alloy are arranged in a direction suitable for said expected
operational direction.
11. A shape memory alloy as set forth in claim 10, wherein the
average grain diameter of crystals is 10 microns or less.
12. A shape memory alloy as set forth in claim 10, wherein said
expected operational direction is a tensile direction.
13. A shape memory alloy as set forth in claim 10, wherein said
expected operational direction is a torsion direction.
14. A shape memory alloy as set forth in claim 10, wherein said
shape memory alloy is in a form of a wire.
15. A shape memory alloy as set forth in claim 10, wherein said
shape memory alloy is an intermetallic compound.
16. A shape memory alloy as set forth in claim 15, wherein said
shape memory alloy is a Ti--Ni based alloy.
17. A shape memory alloy as set forth in claim 15, wherein said
shape memory alloy is a Ti--Ni--Cu based alloy.
18. A method of making a shape memory alloy that is polycrystalline
and has a substantially uniformly fine-grained crystal structure,
crystal orientations thereof being arranged substantially along a
direction suitable for an expected operational direction wherein
said shape memory alloy is selected from the group consisting of a
wire having a solid round cross section, a plate, and a coil, and
wherein said method comprises the steps of: (a) providing a raw
shape memory alloy having a substantially uniformly fine-grained
crystal structure; and (b) arranging crystal orientations of said
raw shape memory alloy substantially along a direction suitable for
an expected operational direction wherein step (a) comprises the
step of: (c) heating said raw shape memory alloy in an amorphous
state or a state similar thereto to the temperature at which
recrystallization begins or a little above for a short period of
time, with a stress applied to said raw shape memory alloy in said
expected operational direction at least in the stage where a
recovery recrystallization begins, to produce a substantially
uniform fine-grained crystal structure with an anisotropy in said
expected operational direction, while relaxing the internal stress
generated in said raw shape memory alloy in said expected
operational direction; and step (b) comprises the steps of: (d)
subjecting said raw shape memory alloy to a high level of
deformation by means of a stress in said expected operational
direction at a very low temperature at which the austenite phase
does not remain in said raw shape memory alloy so that a slide
deformation is introduced into the crystal grains of said raw shape
memory alloy which have been transformed completely into the
martensite phase within a reversible range along the direction of
said stress; (e) heating said raw shape memory alloy to a
temperature between A.sub.f and the recrystallization temperature
with a stress applied to said raw shape memory alloy in said
expected operational direction so that the directions of reversible
slip motions of the respective crystal grains of said raw shape
memory alloy are arranged in a direction suitable for said expected
operational direction.
19. The method as set forth in claim 18, wherein the average grain
diameter of crystals is 10 microns or less.
20. The method as set forth in claim 18, wherein prior to step (c),
said raw shape memory alloy is subject to a severe cold working so
that the crystal structure thereof is destructed and is brought to
a state similar to an amorphous state.
21. The method as set forth in claim 18, wherein said shape memory
alloy is an intermetallic compound.
22. The method as set forth in claim 21, wherein said shape memory
alloy is a Ti--Ni based alloy.
23. The method as set forth in claim 21, wherein said shape memory
alloy is a Ti--Ni--Cu based alloy.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a shape memory alloy (SMA) suitable for
actuators and a method of treating the same.
2. Related Art
Heretofore, upon treating a raw shape memory alloy so as to make it
suitable for use in actuators, generally it has not been done to
refine crystal grains and control crystal orientations of the raw
shape memory alloy.
On the other hand, in order to use a shape memory alloy, it is
necessary to impart a required shape to the shape memory alloy, and
therefore to perform a heat treatment peculiar to each kind of
shape memory alloy. This heat treatment is called shape memory
treatment and it is necessary to strictly control various
conditions thereof, as it is a very delicate treatment. For
example, the following methods have been well known as shape memory
treatments for common Ti--Ni based shape memory alloys. The first
method, which is referred as medium temperature treatment, is the
one wherein a shape memory alloy is sufficiently work hardened and
then cold worked into a desired shape, and thereafter, held at a
temperature of 400 to 500.degree. C. for a few minutes to several
hours with the desired shape being restrained. The second method,
which is referred as low temperature treatment, is the one wherein
a shape memory alloy is held at a temperature of 800.degree. C. or
above for some time, thereafter rapidly cooled and cold worked into
a desired shape, and then held at a low temperature of 200 to
300.degree. C. with the desired shape being restrained (Illustrated
idea collection of applications of shape memory alloys in the
latest patents, written and edited by Shoji Ishikawa, Sadao Kinashi
and Manabu Miwa, published by Kogyo-chousa-kai, pp. 30).
In general, conventional shape memory alloys suffer from the
following shortcomings when used in actuators.
(a) The response characteristic (speed) is inferior.
(b) Usable temperature range is restricted, since M.sub.s and
M.sub.f points (M.sub.s being the temperature at which the
martensite phase transformation starts and M.sub.f being the
temperature at which the martensite phase transformation ends) are
difficult to be raised.
(c) Only a small force can be effectively extracted from the shape
memory alloy.
(d) The service life before being broken is short.
(e) The shape memory alloy tends to lose the memory of an imparted
configuration and permanent strain tends to be produced in the
shape memory alloy for a short period of time.
(f) The strain which can be extracted from the shape memory alloy
as a movement (hereinafter referred as operational strain) is
decreased for a short period of time.
(g) Shape memory alloy materials, such as Ti--Ni based or
Ti--Ni--Cu based alloys and the like, which are intermetallic
compounds having strong covalent bonding characteristic and are
difficult to work, are difficult to use when they are in certain
compositions, since they are very brittle and fragile.
With such shortcomings, 80 to 90% or more of applications of shape
memory alloys have been those wherein they are used as superelastic
spring materials and only the rest has been directed to actuators.
Moreover, most of the shape memory alloys for use in actuators have
been formed into the shape of a coil spring, wire or plate and have
been expected to be reverted from a configuration deformed by
bending or twisting and bending to the original configuration upon
application of heat (in case the shape memory alloy is formed into
a coil spring shape, though macroscopically or apparently it is
deformed as if it were elongated or compressed upon application of
a force thereto, in a true sense the deformation it is subject to
is a twisting and bending one). The reason for utilizing reversion
from a bending deformation or twisting and bending deformation as
stated above has been that the shape memory alloy should be used so
that its small strains may be multiplied since the range of its
shape memory effect (SME) stably available is very narrow. Though
it is said that, in conventional shape memory alloys, the maximum
operational strain reaches a few percent to about 10 percent, this
is true only when deformation and shape recovery are performed only
once or a few times. Practically speaking, when deformation and
shape recovery are repeated over large cycle numbers with regard to
the conventional shape memory alloy, the operational strain is
decreased and the alloy loses the memory of the imparted
configuration and eventually is broken.
All of the conventional shape memory treatments intend to keep the
shape stability while obtaining the pseudoelasticity and shape
memory effect by partly producing microstructures which can cause
pseudoelasticity and shape memory effect in microstructures of the
shape memory alloy strengthened by work hardening. In other words
all of the conventional shape memory treatments are those which
obliges to sacrifice pseudoelasticity and shape memory effect to
some extent to keep shape stability.
On the other hand, the present inventor has disclosed in U.S. Pat.
No. 4,919,177 a method of treating Ti--Ni based shape memory alloy
wherein a Ti--Ni based polycrystalline shape memory alloy material
is subjected to a heat cycle which rises and drops over the
transformation region of the shape memory alloy as well as to a
directional energy field. According to this method, the crystal
orientations of the shape memory alloy are rearranged along a
specific direction and the disadvantages of the conventional shape
memory alloy are overcome considerably.
However, in the method disclosed by the present inventor, the
crystal grains of the shape memory alloy are not refined but caused
to grow in size. Besides, since a tensile force is applied to the
shape memory alloy in the final step of arranging the crystal
orientations, there is a tendency that the microstructure of the
shape memory alloy finally obtained is destroyed by the tensile
force. Therefore, it is still not enough in overcoming the
disadvantages of the conventional shape memory alloy.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a
shape memory alloy having a good response characteristic and a
method of treating a shape memory alloy for obtaining such a shape
memory alloy.
It is another object of the present invention to provide a shape
memory alloy which can be used over a wide range of temperature and
a method of treating a shape memory alloy for obtaining such a
shape memory alloy.
It is still another object of the present invention to provide a
shape memory alloy from which a greater force can be practically
and effectively extracted and a method of treating a shape memory
alloy for obtaining such a shape memory alloy.
It is a further object of the present invention to provide a shape
memory alloy from which great operational strains can be extracted
over large cycle numbers and a method of treating a shape memory
alloy for obtaining such a shape memory alloy.
It is a still further object of the present invention to provide a
shape memory alloy exhibiting a huge two-way shape memory effect
(reversible shape memory effect) and a method of treating a shape
memory alloy for obtaining such a shape memory alloy.
It is another object of the present invention to provide a shape
memory alloy having a long service life and a method of treating a
shape memory alloy for obtaining such a shape memory alloy.
It is still another object of the present invention to provide a
shape memory alloy which does not lose its memorized shape easily
and a method of treating a shape memory alloy for obtaining such a
shape memory alloy.
It is a further object of the present invention to provide a shape
memory alloy of which operational strain diminishes less even with
an increase of a deformation-recovery cycle number and a method of
treating a shape memory alloy for obtaining such a shape memory
alloy.
It is a still further object of the present invention to provide a
shape memory alloy which exhibits stably the aforesaid various
excellent properties over large cycle numbers for a long period of
time and a method of treating a shape memory alloy for obtaining
such a shape memory alloy.
It is another object of the present invention to provide a method
of treating a shape memory alloy which makes it possible to employ,
as raw materials, those shape memory materials which have been
regarded as difficult to use because of their brittleness and
easiness to crack and convert them into ductile shape memory alloys
in the shape of a wire or sheet etc.
It is yet another object of the present invention to provide a
method of treating a shape memory alloy which makes it possible to
arrange crystal orientations of a shape memory alloy without
damaging the microstructure of the alloy.
Crystal grains of a shape memory alloy have orientations and there
exist a plurality of orientations along which reversible slips or
shearing deformations (variants), wherein microscopically relative
moving ranges between the atoms of the alloy are restricted, can
appear, though they are limited in number. For example, in case of
a Ti--Ni based shape memory alloy, there are as much as twenty four
(24) orientations along which the deformations referred to as
variants can occur. In the present invention, the crystal
orientations of the shape memory alloy are arranged substantially
along a direction suitable for an expected operational direction,
in other words, a direction suitable for a movement of the shape
memory alloy in the expected operational direction. The term
"expected operational direction" as herein used means a direction
such as a tensile or twisting direction or the like in which the
shape memory alloy is expected to move when used in an actuator
after the completion of the treatment. For example, when a shape
memory alloy in the wire shape is used in a contraction-relaxation
fashion, the expected operational direction is a tensile direction,
while when a shape memory alloy in the coil spring shape is used,
the expected operational direction is a torsion direction. (In case
a shape memory alloy in the coil spring shape is used, it performs
shape recovery from a twisting and bending deformation upon
heating. Therefore, strictly speaking, it may be said that the
expected operational direction in this case is a torsion and
bending direction. However, the substantial expected operational
direction is a torsion direction, because bending deformation
comprises a negligible percentage.)
A method of treating a shape memory alloy in accordance with the
present invention comprising the steps of providing a raw shape
memory alloy with a substantially uniformly fine-grained crystal
structure; and arranging crystal orientations of the raw shape
memory alloy substantially along a direction suitable for an
expected operational direction.
It is preferred that the average grain size of the raw shape memory
alloy is selected to be 10 microns or less in the step of providing
the raw shape memory alloy with a substantially uniformly
fine-grained crystal structure. Most preferred is the average grain
size in the range of 1 micron to several microns or less. With such
grain size, the shape memory alloy after the completion of the
treatment is particularly stable when subjected to
deformation-recovery cycle.
In general, specific characteristic properties of crystalline
materials are based on the phenomena in crystal grains of the
materials. Accordingly, in many cases, these specific
characteristic properties should naturally be most remarkably
recognized when the materials are of single crystal. For this
reason, when the excellent properties or functions of some material
are to be utilized, in general, the best results can be obtained
when the material is of single crystal. Basically the shape memory
alloy is no exception on this matter. A shape memory alloy of
single crystal can be deformed in a slip direction by very small
force in the range where reversible slip deformation can occur
under a low temperature at which it is in the martensitic phase as
a whole (Slip deformation in this specification means shearing
deformation which is the cause of the shape memory effect and
wherein reversible movement is possible within a limited range, but
it does not mean permanent and continuous slip between atoms which
is the cause of the plastic deformation).
However, in practice it is extremely difficult to industrially
produce the single crystalline material, and the production of it,
even when achieved, should be very expensive. Besides, in case of a
shape memory alloy, when it is of single crystal, its
microstructure becomes unstable.
Of course conventional shape memory alloys are polycrystalline
substances, and in general, orientations of the respective crystals
thereof have been random and the grain sizes of respective crystals
are uneven, and thereby it is thought that aforesaid various
shortcomings are caused (this will be discussed later in
detail).
The present inventor has found that a shape memory alloy can be
obtained which has both advantages of the single crystalline shape
memory alloy and those of the conventional polycrystal shape memory
alloys, when the shape memory alloy, as in the present invention,
is formed of a polycrystal material and provided with a
substantially uniformly fine-grained crystal structure, and the
crystal orientations thereof are arranged along a direction
suitable for an expected operational direction. When the crystal
grain sizes of the alloy are made substantially uniform and the
crystal orientations are arranged along a direction suited for a
desired movement of the alloy, even if gigantic shape recovery
force is produced in respective crystal grains, no part of the
alloy is subject to an excessive deformation and the internal
structure of the alloy is difficult to destroy. Besides, when
respective crystal grains are adequately small, structural
contradictions caused by differences between deformation directions
of the respective crystal grains, etc. are also small and thereby
the respective crystal grains themselves are difficult to destroy.
Moreover, in such a material, since the volume proportion of the
structure at and around the crystal grain boundaries to that within
the grains is comparatively larger, its ability to absorb the
structural contradictions is high. Further, such a material can be
reformed into a shape memory alloy in the shape of a wire or sheet
etc. which is sufficiently ductile over a wide strain range, even
in the case where it is brittle when it is a raw material. The
reason for this is presumed that, in such a material, the structure
at and around the crystal grain boundaries exhibits properties like
those of an amorphous material. Even the respective crystal grains
are fine, if the crystal orientations are arranged, comparatively
large shape memory effect can be extracted from the shape memory
alloy. A force required to deform the shape alloy is small, since
the orientations of the respective crystals along which the
crystals are easy to move are arranged in the same direction.
Because the volume proportion of the structure at and around the
crystal grain boundaries to that within the grains is comparatively
larger, large elastic energy can be stored at and around the
crystal grain boundaries without employing the measures of
depositing impurities there, or the like, and thereby a stable and
large two-way shape memory effect can be obtained as well as the
property that a force required to deform the alloy is small.
Thus, the shape memory alloy in accordance with the present
invention has the following excellent properties, though some of
them have been already mentioned above.
(A) Since the temperature hysteresis is small on the
temperature-stress diagram and the transformation temperature range
is narrow, heating and cooling of the alloy can be taken place
quickly, the response of the alloy is good, and a high-speed
reciprocating motion can be achieved. For example, when applied to
a Ti--Ni--Cu based shape memory alloy, the temperature hysteresis
can be almost zero over a comparatively wide range. A successive
reciprocating operational strain reaching to almost 80% of that in
the full stroke (strain .epsilon.=4%) could be successfully
extracted from a shape memory alloy in accordance with the present
invention with a 150 Mpa load and a temperature difference of
merely 10.degree. C. This, when compared to a engine, is equivalent
to the revolution speed is remarkably raised with the same size.
Accordingly, it is equivalent to that the horsepower as well as the
load capacity is considerably raised. A significant improvement of
the responsiveness can be expected, when used in a mechanism such
as a servo actuator wherein two-way movement is required.
(B) The force which can be practically extracted from the shape
memory alloy (hereinafter referred as recovery force) can be
increased. The recovery force does not depend on the maximum
recovery stress but the limit of the stress repeatedly usable in
consideration of fatigue of the alloy, etc. When compared to a
engine or motor, the recovery force corresponds to the maximum
torque. With the shape memory alloy treated by the method in
accordance with the present invention, the limit of the stress
practically available in the reiterative operation is high, even
when the maximum recovery stress is same as that of the
conventional shape memory alloy. The conventional shape memory
alloy has a small recovery force, and if operated repeatedly with
an excessively large stress applied thereto, it suffers from loss
of the memory of the imparted configuration, decrease of the
operational strain and rupture, as stated above. It means
shortening of service life of the actuator. This is the reason why
most of conventional shape memory alloy actuators have been formed
in the shape of a coil spring, as previously stated. With the coil
spring shape, the strain produced in the alloy is very small when
the alloy is deformed. Therefore, the stress actually used has been
considerably small as compared with the maximum stress practically
available.
(C) Large operational strains can be extracted over large cycle
numbers. The shape memory alloy in accordance with the present
invention, when formed into a rectilinear shape, can achieve a
deformation-shape recovery cycle with a tensile strain of 5% or
more. The value of the operational strain, 5% or more stands
comparison with that a 1 m long round bar is expanded and
contracted by 5 cm or more. This magnitude of strain is much larger
than that of the strain which an ordinary coil spring is subject to
when it is deformed and restored between the coil and rectilinear
shape. This value is much larger than the ranges of strains
available in case of conventional shape memory alloys including
superelastic alloys. When the treatment in accordance with the
present invention is applied to a brittle raw shape memory alloy
material such as Ti--Ni--Cu based shape memory alloy and the like,
huge operational strains as stated above can be extracted stably
over more than one hundred million cycles. When the conventional
shape memory alloy is used in the coil spring shape, in most cases,
the moving strain is less than 0.1% in tensile strain equivalent.
In other words, in most cases, the coil spring of a shape memory
alloy has been used with almost the same magnitude of displacement
as the coil spring of a non-shape memory alloy metal such as iron
and the like.
(D) It is possible to cause a shape memory alloy to exhibit a huge
two-way shape memory alloy effect. The two-way shape memory effect
is a phenomenon wherein a shape memory alloy recovers the original
configuration upon heating and deforms into another configuration
upon cooling, and no force or only a very small force is required
when the alloy is subject to the deformation at a low temperature
in a direction opposite to the shape recovery. Apparently, it
appears that the shape memory alloy remembers two configurations,
the deformed configuration at a low temperature and the original
configuration at a high temperature. For instance, in case the
shape memory alloy is rectilinear and the deformed configuration
(length) thereof is the one stretched from the original
configuration (length), the shape memory alloy contracts to the
original length and becomes hard upon heating, while it extends by
itself to the deformed length and becomes soft just like a muscle
relaxes upon cooling, even in the absence of a load. In other
words, the shape memory alloy expands and contracts, driven by
heating and cooling alone in the absence of a bias force from the
outside. According to literature, etc., it has been thought that,
generally the two-way shape memory effect is a phenomenon observed
only within the range wherein a strain .epsilon. is 1% or less in
tensile strain equivalent and it is difficult to put it to
practical use since it is unstable. In fact, hitherto devices
utilizing the two-way shape memory effect have been hardly
found.
According to the present invention, however, it is possible to
cause a huge two-way shape memory effect almost over the whole
range wherein the shape memory effect occurs, namely, the whole
range of recoverable strain. According to the present invention,
the two-way shape memory effect with a strain of 5% can be
exhibited even in the absence of a load. The present inventor
postulates that, since the polycrystal shape memory alloy in
accordance with the present invention has crystals each of which
orientation, size and position are adapted to deformations from the
outside, a stable two-way shape memory effect can be induced almost
in the whole range of the operational strain, if there exists in
the alloy the slightest level of a residual stress field resulted
from the working in a direction opposite to the shape recovery
direction. This huge two-way shape memory effect appears stably
over about one hundred million cycles in the absence of a load.
(E) The shape memory alloy in accordance with the present invention
has a long service life. The conventional shape memory alloy has a
service life of about one hundred thousand cycles, at the largest,
even with the small operational strain. Particularly, in case a
movement wherein the operational strain exceeds 2% in tensile
strain equivalent is performed, there is a tendency that its
service life becomes extremely short. However, the shape memory
alloy in accordance with the present invention provides a stable
movement over one hundred million cycles with a huge operational
strain reaching nearly 5%.
(F) The memory of the imparted configuration and the range of the
operational strain are stable, that is, the memory of the imparted
configuration and the range of the operational strain do not
diminish with cycle number of the deformation and recovery or do
only slightly. In other words, the magnitude of the operational
strain has little effect on the service life of the shape memory
alloy. The reason for it is postulated that the shape memory alloy
in accordance with the present invention has the orientations,
sizes and arrangements of the respective crystals in a state
adapted to deformations from the outside. It is presumed that the
deformation from the outside is undertaken, to a certain extent,
mainly by the crystals which achieve a huge reversible
thermo-elastic deformation that is characteristic of a shape memory
alloy, while the deformation larger than it is undertaken by the
structure at and around the crystal grain boundaries wherein a
reversible thermo-elastic deformation is hardly produced. With such
structure of the shape memory alloy, displacements, plastic
deformations and rotations of the respective crystal grains are
hard to occur even with large cycle numbers, and the alloy is
hardly subject to a permanent deformation.
(G) Even when the raw material is brittle, it can be reformed into
a ductile shape memory alloy in the shape of a wire, sheet or the
like. The shape memory alloy in accordance with the present
invention has higher apparent ductility than shape memory alloys
treated by the conventional shape memory treatment since it
consists of the fine crystal grains reversibly deformable and the
structure at and around the crystal grain boundaries which exhibits
amorphous like properties and occupies a considerable part of the
alloy with regard to volume.
(H) The various excellent properties of the shape memory alloy
mentioned above are stable over a long time of period and large
cycle numbers.
In a particular aspect of the method of treating a shape memory
alloy in accordance with the present invention, the step of
providing a raw shape memory alloy having a substantially uniformly
fine-grained crystal structure comprises the steps of heating the
raw shape memory alloy in an amorphous state or a state similar
thereto to the temperature at which recrystallization begins or a
little above for a short period of time, with a stress applied to
the raw shape memory alloy in the expected operational direction at
least in the stage where a recovery recrystallization begins, to
produce a substantially uniform fine-grained crystal structure with
an anisotropy in the expected operational direction, while relaxing
the internal stress generated in the raw shape memory alloy in the
expected operational direction.
In case the raw shape memory alloy is not in an amorphous state or
a state similar thereto, the raw shape memory alloy can be be put
into a state similar to amorphous state, for instance, by being
subject to a severe cold working. It is preferable that the severe
cold working is achieved at a cryogenic temperature which is
sufficiently lower than the temperature singular point B of the raw
shape memory alloy. The point B is an inflection point observed in
the sub-zero temperature range and is associated with transitions
of the physical property values such as specific heat, electrical
resistance and the like (This will be explained later in more
detail). The object for this is to completely transform
non-martensite structures remaining in the alloy, even if the
amount of them are very small, into the martensite. In general, the
so called martensite finished point M.sub.f at which the shape
memory alloy transforms completely from austenite to martensite is
the temperature which is measured with respect to a specimen
completely annealed. In worked materials, however, there remain a
considerable amount of the non-martensite structures even at this
temperature. The non-martensite structures may be retained
austenite, a structure resulted from work hardening or the
like.
Upon heating the raw shape memory alloy to the temperature at which
recrystallization begins or a little above for a short period of
time, the raw shape memory alloy may be either in a state where a
stress is applied to it in the expected operational direction or
where it is constrained in a shape not loosened in the absence of a
load. At this stage, since the raw shape memory alloy has a
martensitic component which can recover the shape in the expected
operational direction upon heating, if it is constrained in a shape
not loosened in the absence of a load, a stress is produced in the
expected operational direction while heating and thereby the same
result is obtained as when the alloy is constrained with a stress
applied thereto prior to heating as stated above. What is essential
is that at least when a recovery recrystallization begins the raw
shape memory alloy is in a state where a stress is loaded thereto
in the expected operational direction.
In the particular aspect of the method of treating a shape memory
alloy in accordance with the present invention, the step of
arranging crystal orientations of the raw shape memory alloy
comprises the steps of subjecting the raw shape memory alloy to a
high level of deformation by means of a stress in the expected
operational direction at a very low temperature at which the
austenite phase does not remain in the raw shape memory alloy so
that a slide deformation is introduced into the crystal grains of
the raw shape memory alloy which have been transformed completely
into the martensite phase, within a reversible range along the
direction of the stress; heating the raw shape memory alloy to a
temperature between A.sub.f (a temperature at which the austenitic
transformation ends) and the recrystallization temperature with a
stress applied to said raw shape memory alloy in the expected
operational direction so that the directions of reversible slip
motions of the respective crystal grains of said raw shape memory
alloy are arranged in the expected operational direction.
The crystal orientations of the raw shape memory alloy are arranged
when the directions of reversible slip motions of the respective
crystal grains are arranged in the expected operational direction.
Hereupon, the orientation of crystal grain means the one where a
reversible slip deformation due to the martensitic transformation
is easy to occur practically such as one of orientations of
variants and the like, but not necessarily one and the same
orientation from the view point of the crystallography.
The step of introducing a slide deformation to the crystal grains
and that of arranging the directions of reversible slip motions of
the s crystal grains may be repeated a required number of times.
Generally it suffices to repeat one to three times.
In the method of treating a shape memory alloy in accordance with
the present invention, it is preferable to take place a step of
running-in, after having rearranged the crystal grains of the raw
shape memory alloy along the direction which is suited for the
reversible deformation of the alloy in the expected operational
direction as stated above, in order to obviate instability of the
alloy which appears in the initial stage of its repetition
movement. This running-in step is a process which aims for the same
effect as the training process which has been employed in the
conventional shape memory treatment.
Preferably, the running-in step is performed, after arranging the
directions of reversible slip motions of the respective crystal
grains of the raw shape memory alloy in the expected operational
direction, by subjecting the raw shape memory alloy to a heat cycle
between a temperature of M.sub.f point or below and a temperature
at which only a high level of plastic deformation is relaxed, while
controlling a stress applied to the raw shape memory alloy without
restraining the strain introduced in the raw shape memory alloy. In
general, it is preferable that a few to several tens cycles of the
heat cycle is applied to the raw shape memory alloy. In accordance
with the running-step, a work hardening and a structural defect
having an elastic energy field which contribute to the dimensional
stability and two-way shape memory effect of the alloy can be
stored in the microstructure at and around the crystal grain
boundaries to the desired degree and thereby the instability of the
alloy which appears in the initial stage of its repetition movement
can be dissolved.
It has not been yet fully elucidated theoretically what phenomenon
occurs in the shape memory alloy and why the alloy exhibits various
excellent properties as stated above when the treatment in
accordance with the present invention is carried out. However, to
make the present invention easily understood, a supplementary
explanation will be given hereunder on the basis of a hypothesis
the present inventor holds at present.
It is considered that in a polycrystalline shape memory alloy each
crystal performs as a single crystal, while the structure at and
around the crystal grain boundaries connects the crystals with each
other. Therefore, in case orientations and sizes of the crystals
are random, when the respective crystals present large deformations
due to the superelasticity and shape memory effect, the structure
at and around the crystal grain boundaries is subject to structural
contradictions caused by the deformations of the crystals. The
conventional shape memory alloy, treated with an ordinary shape
memory treatment after manufactured by ordinary working such as
casting, hot working and the like, is polycrystalline and random in
the crystal orientations and sizes thereof, and some of the
crystals thereof have been destroyed by strong working. Such
circumstances constitute obstacles disturbing a smooth deformation
and shape recovery of the alloy, and thereby a considerable force
is required to deform the alloy, even when at a temperature
sufficiently low for the martensitic transformation to be
completed. Therefore, satisfactory shape memory effect can not be
achieved when it is used as an actuator, even after the shape
memory treatment.
The shape recovery force within the crystal grain is strong and has
enough magnitude to deform plastically and destroy the structure at
and around the crystal grain boundaries which constitutes a
connection between crystal grains and the crystal grains which is
not yet in the shape recovery state. This may explains the reason
why the conventional shape memory alloy soon loses the memory of
the imparted shape and becomes hard, with the operational strain
thereof decreasing, when it is subject to repetitions of a large
deformation and shape recovery. It may be because the interior of
the shape memory alloy is changed little by little due to the great
shape recovery force. Especially, in the case where the shape
memory alloy performs the shape recovery when it is subject to a
large deformation and restrained in the deformed configuration, the
shape recovery forces of the respective crystal grains act on the
interior of the alloy material at a stretch and the shape memory
alloy deteriorates rapidly. The fact is that, in case of the
conventional shape memory alloy, the superelastic spring and the
like, the above-mentioned defect should be covered up by practicing
strong working to cause work hardening in the alloy, and
consequently constructing the internal structure in the alloy where
the huge shape recovery forces of the crystals are restrained.
On the other hand, in accordance with the present invention, the
sizes of the crystal grains being made even and the orientations
thereof being arranged along the predetermined direction, even if a
huge shape recovery force is produced in each crystal grain, there
is no part in the alloy where an excessive deformation is produced
and the internal structure of the alloy becomes hard to break.
Besides, if the respective crystal grains are adequately fine,
structural contradictions produced due to the differences between
the orientations of the respective crystal grains or the like are
small, and the crystal themselves becomes hard to break. Moreover,
in such a fine-grained material, since the volume proportion of the
structure at and around the crystal grain boundaries to that within
the grains is comparatively larger, the ability to absorb the
structural contradictions is high. Further, probably as the
structure at and around the crystal grain boundaries exhibits
properties like those of an amorphous material, it can be converted
into a shape memory alloy in the shape of a wire or sheet, etc.
which is sufficiently ductile over a wide strain range, even in the
case where it is brittle as a raw material. Though the respective
crystal grains are fine, since the crystal orientations are
arranged along the specific direction, a comparatively large shape
memory effect can be extracted from the shape memory alloy. The
force required to deform the shape alloy is small, since the
orientations of the respective crystals along which they are easy
to move are arranged along the specific direction. Because the
volume proportion of the structure at and around the crystal grain
boundaries to that within the grains is comparatively larger, large
elastic energy can be stored at and around the crystal grain
boundaries without employing the measures of depositing impurities
there, or the like, and thereby a stable and large two-way shape
memory effect can be obtained as well as the property that a force
required to deform the alloy is small.
When crystal orientations of a shape memory alloy are random, the
larger the average grain size of the shape memory alloy is, more
conspicuously the shape memory effect occurs. However, in that
case, stability as a material is deteriorated. The reason for it is
thought that structural contradictions are liable to be produced in
the alloy due to the large grain sizes and random crystal
orientations, causing changes of structure in the alloy. For
instance, a treatment for a shape memory alloy generally called
high temperature treatment has been known wherein the shape memory
alloy is annealed sufficiently at a high temperature. According to
this treatment, because the crystal grain sizes become larger, a
large shape memory effect can be induced, but loss of the memorized
shape, generation of a permanent deformation and decrease of the
operational strain, etc. are caused soon with a
deformation-recovery cycle number. Accordingly, though large
operational strains can be extracted, the alloy becomes
functionally unstable, and thereby nowadays this high temperature
treatment is not put to practical use. On the contrary, when the
crystal grains are fine, though the magnitude of the shape memory
effect decreases relatively, the shape memory alloy becomes
materially stable, since structural contradictions produced in the
alloy due to the movement of the respective crystals become small
and affect less the respective crystals.
Besides, as stated before, with a fine-grained structure, the
volume proportion of the structure at and around the crystal grain
boundaries to that within the grains is larger, as compared with in
the case of a coarse-grained structure. Accordingly, the properties
of the boundaries of crystal grains appears outside conspicuously.
It is considered that the structure at around the crystal grain
boundaries is in disorder and amorphous like properties are
dominant there, as compared with the interior of the crystal grain
which has a well-ordered atomic arrangement. The metal structure at
and around the crystal grain boundaries and that within the grains
are structurally different material, though they make little
difference in composition. Naturally, the properties of the metal
structure at around the crystal grain boundaries must differs very
markedly from those of the metal structure within the grains. While
it is easy to impart a deformation related to the shape memory
effect to the structure within the crystal grains, it is difficult
to impart such deformation to the structure at around the crystal
grain boundaries, since it is constrained, getting between the
crystal grains, and has poor reversible deformability. Therefore,
it is considered that the metal structure at and around the crystal
grain boundaries and that within the grains are two different
materials. As a matter of course, transformation points within
crystal grains differ from those at and around the crystal grain
boundaries. It is thought that the process of rearranging the
crystal orientations along the specific direction in the present
invention uses the aforesaid properties of the crystal grain
boundaries and therearound.
Most of conventional shape memory alloy production methods and
shape memory treatments control strains of the shape memory alloy
to define a shape of a finished shape memory alloy and a memorized
shape. On the contrary, one of the distinguishing characteristics
of the present invention is that most of the main processes thereof
are carried out in a state where not the strain but the stress is
controlled, allowing the raw shape memory alloy to deform freely.
By not controlling the strain, the present invention utilizes the
property of the shape memory alloy that the alloy itself
reconstructs the internal structure thereof to be adapted for the
movement circumstances thereof.
Besides, since the entire treatment process is carried out in rapid
dynamic heating and cooling operations, long spells of heat
treatment is not required unlike in the case of conventional
treatments, though the procedure is comparatively complicated.
Therefore, a high speed and consecutive large-scale process for
treating a shape memory alloy material can be attained which
provides a high-performance shape memory alloy.
Shape memory alloys, more particularly Ti--Ni based and Ti--Ni--Cu
based shape memory alloys are not ordinary alloys consisting of two
or more metals simply mixed together but intermetallic compounds
having strong covalent bonding character. Due to the strong
covalent bonding character, they have characteristics like those of
inorganic compounds such as ceramic and the like, though being
metal. Free electrons are restrained considerably within them
because of the strong covalent bonding as compared with the case
with metallic bond. Smallness of the free electron movement within
them is supported by their properties of poor heat conduction and
high electric resistance, though they are metal. The difficulty of
free electron movement makes it hard for the fusion and
reorganization of the electron cloud to occur. This is a strong
reason that Ti--Ni and Ti--Ni--Cu based shape memory alloys are
brittle materials which are hard to plastically deform. Though the
treatment in accordance with the present invention can be applied
to all kinds of shape memory alloys, particularly it is very
effective when applied to shape memory alloys, such as Ti--Ni or
Ti--Ni--Cu based shape memory alloys or the like, which have strong
covalent bonding character and are brittle as raw materials. When
the treatment is applied to such materials, the service life, the
moving range and the dimensional stability thereof are remarkably
improved especially in repetition action under a heavy load, and
the ductility thereof is also improved. Moreover, it becomes
possible to use alloy compositions which hitherto have been
considered to be no use for shape memory alloys, as alloys with
them being hard to work or being too brittle even though possible
to be worked. Accordingly, it can be expected to create new shape
memory alloys which have unprecedented properties.
In another particular aspect of the method of treating a shape
memory alloy in accordance with the present invention comprises the
steps of subjecting a raw shape memory alloy having an anisotropy
in an expected operational direction to a high level of deformation
by means of a stress in the expected operational direction at a
very low temperature at which the austenite phase does not remain
in the raw shape memory alloy so that a slide deformation is
introduced into the crystal grains of the raw shape memory alloy
which have been transformed completely into the martensite phase,
within a reversible range along the direction of the stress;
heating the raw shape memory alloy to a temperature between A.sub.f
and the recrystallization temperature with a stress applied to said
raw shape memory alloy in the expected operational direction so
that the directions of reversible slip motions of the respective
crystal grains of the raw shape memory alloy are arranged in the
expected operational direction.
In this case, the raw shape memory alloy is not necessarily should
have substantially uniformly fine-grained crystal structure.
According to this aspect, also the crystal orientations are
arranged along the direction suitable for the expected operational
direction without breaking the structure of the shape memory alloy,
as in the aforesaid aspect.
Further scope of applicability of the present invention will become
apparent from the detailed description given hereinafter. However,
it should be understood that the detailed description and specific
examples, while indicating preferred embodiments of the invention,
are given by way of illustration only, since various changes and
modifications within the spirit and scope of the invention will
become apparent to those skilled in the art from this detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and the other objects, features and advantages of the
present invention will become apparent from the following detailed
description when taken in connection with the accompanying
drawings. It is to be understood that the drawings are designated
for the purpose of illustration only and are not intended as
defining the limits of the invention.
FIG. 1 is a schematic presentation of transformation points and
temperature singular points of a raw shape memory alloy in a first
embodiment of the treatment in accordance with the present
invention.
FIG. 2 is a presentation of the transformation points and the
temperature singular point S, etc. of a Ti--Ni--Cu based shape
memory alloy appearing upon heating which are actually measured
with a DSC (Differential scanning calorimeter).
FIG. 3 is a presentation of the cryogenic temperature singular
point B of a Ti--Ni--Cu based shape memory alloy actually measured
with a DSC.
FIG. 4 is a cross-sectional view showing step 1 of the first
embodiment.
FIG. 5 is a cross-sectional view showing step 2 of the first
embodiment.
FIG. 6 is a cross-sectional view showing step 3 of the first
embodiment.
FIG. 7 is an example of stress-strain diagram in the step 3 of the
first embodiment.
FIG. 8 is a cross-sectional view showing step 4 of the first
embodiment.
FIG. 9 is a presentation of the comparison of the stress-strain
curve of the shape memory alloy obtained by the first embodiment
with those of conventional shape memory alloys.
FIG. 10 is a explanatory drawing showing the test condition for
measuring the characteristics of FIG. 9.
FIG. 11 is a perspective view showing a state where a raw shape
memory alloy is subject to a twisting deformation in step 2 of a
second embodiment of the treatment in accordance with the present
invention.
FIG. 12 is a cross-sectional view showing a state where the raw
shape memory alloy torsionally deformed in the step 2 of the second
embodiment is heated under restraint.
FIG. 13 is a perspective view showing step 3 of the second
embodiment.
FIG. 14 is a perspective view showing step 4 of the second
embodiment.
FIG. 15 is a perspective view showing step 5 of the second
embodiment.
FIG. 16 is a perspective view showing step 6 of the second
embodiment.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will hereunder be described in conjunction
with preferred embodiments of the invention which are shown in the
drawings. In the drawings like reference numerals are used
throughout the various views to designate like parts.
FIGS. 4 through 9 show a first embodiment of the method of treating
a shape memory alloy in accordance with the present invention. In
this embodiment, it is expected that upon using the finished shape
memory alloy, the alloy is contracted to a memorized length, namely
original length upon heating, while it relaxes upon cooling,
expanding to a original deformed length, that is, a length with an
elongation deformation from the memorized length. Therefore, the
expected operational direction is a tensile direction in this
embodiment. In this embodiment, a Ti--Ni based shape memory alloy
material and a Ti--Ni--Cu based shape memory alloy material
containing 8 to 12 atomic percent Cu are used as raw shape memory
alloys 1.
The treatment in this embodiment basically consists of three
stages. The first stage (Steps 1 and 2) is a process of producing
fine-grained anisotropic crystals. The second stage (Steps 3 to 5)
is a process of rearranging the respective crystals to conform to
the expected operational direction of the alloy. The third stage
(Step 6) is a running-in process of dissolving instability of the
alloy which appears in the beginning of the reiterative operation.
However, the essence of the treatment resides in the first and
second stages. Upon completion of the second stage, a high
performance shape memory alloy for actuators is already obtained.
Hereunder the treatment of this embodiment will be explained in
order.
(Preparatory operation)
Raw shape memory alloy materials manufactured by casting and hot
working are annealed, and thereafter worked into a desired size by
drawing with a die or cold rolling. From the worked shape memory
alloys raw material specimens H which are left as work-hardned and
canonical specimens N which are annealed sufficiently at about
900.degree. C. in accordance with JIS (Japanese Industrial
Standard) are prepared. The specimens H and N are subject to a
consecutive and slow heat cycle, and changes of their specific
heat, electrical resistance, size, hardness, structure and the like
are observed, respectively, and the transformation points and
singular points of the raw shape memory alloys are measured. FIG. 1
schematically shows the general relationships between the
transformation points and the singular points of the raw shape
memory alloys. The numeric values in the figure represent only a
rough standard. The temperatures of the transformation points and
singular points vary considerably according to kinds of raw shape
memory alloys. FIGS. 2 and 3 shows examples of actual measurement
data of DSC.
With regard to the temperature range of heat cycle used for the
measurement, the maximum heating temperature is selected to be
about 800.degree. C. and the minimum cooling temperature is
selected to be -196.degree. C. which is the temperature of liquid
nitrogen. From the specimen H as work-hardened, mainly the
temperature singular point S and the recrystallization temperature
R are observed. Here, the temperature singular point S is an
inflection point of physical properties representing
transformations such as the specific heat, electrical resistance,
hardness and the like which is observed between the temperature
range D where a high level of plastic deformations are relaxed and
the recrystallization temperature R (the temperature range D will
be discussed later in more detail). At present, the inventor
considers that this temperature singular point S is associated with
the transformation of crystal grain boundaries. From the specimen N
with which the recrystallization is performed by heating, the
temperature singular point B is observed as well as the
transformation points A.sub.s, A.sub.f, M.sub.s and M.sub.f which
are associated with the shape memory effect. The temperature range
D where only a high level of plastic deformation is relaxed is
observed as the difference in the specific heat between the
specimens N and H. The temperature singular point B is an
inflection point of physical properties representing
transformations such as the specific heat, electrical resistance
and the like which is observed in the sub-zero temperature range
and considered as a transformation point in the sub-zero
temperature range. Though also in the specimen H, sometimes such
singular point is observed, it is not distinct as in the case with
the specimen N and its temperature is liable to differs a little
from that observed in the specimen H, perhaps due to the internal
stress. Therefore, as the transformation points, those observed in
the specimen N are employed, except the temperature singular point
S, the temperature range D and the recrystallization temperature
R.
Though the temperature singular point B varies with the composition
of alloys, in most cases it exists in a very low temperature range
of -40.degree. C. to -150.degree. C. which is difficult to obtain
without liquid nitrogen or the like. Accordingly, it is difficult
to find the temperature singular point B under an ordinary
metallurgical measurement environment. In some conditions of
materials the temperature singular point B cannot be confirmed
clearly. Accordingly, there is very little literature which refers
to it. However, this temperature singular point B is an
particularly important temperature in this embodiment. It seems
that the M.sub.f point measured with the DSC, etc. is principally
that of the interior of the grains which occupy the great portion
of the crystals of the raw alloy. However, since the crystal grain
boundaries are restrained between crystals having different
orientations, it is considered that even on the M.sub.f temperature
or below there still exists a component which remains as in a state
near austenite phase, namely the retained austenite phase. Besides,
since the elastic energy level at the crystal grain boundaries can
be high because of work hardening due to plastic deformations and
depositions of impurities which are peculiar to the crystal grain
boundaries, it is no wonder that the M.sub.f point of the structure
at and around grain boundaries alone lies at a lower temperature.
The present inventor thinks that the temperature singular point B
which is much lower than the M.sub.f point measured with the DSC is
a M.sub.f -point-like transformation point of the structure at and
around the crystal grain boundaries. According to the data measured
with the DSC, in most cases the respective transformation points
and temperature singular points appear as gently-sloping inflection
points and it is rarely the case that they have a distinct peak.
The reason for this is thought that the raw alloys measured are
polycrystalline substances each having crystals which are divergent
in their sizes, orientations and conditions under which they are
constrained. As a matter of fact, the temperatures which are
commonly called transformation points are also represented by the
central or average values of transformation temperature ranges
having a certain width, respectively.
(Step 1)
A raw shape memory alloy material 1 manufactured by casting and hot
working is annealed, and thereafter is subject to a high level of
deformation so as to be formed into a wire shape by cold working,
in such a manner that a great deformation extends sufficiently to
the interior thereof and an anisotropy in the tensile direction
remains therein. To be concrete, as shown in FIG. 4, the raw shape
memory alloy 1 is subject to wire drawing with a die 2, repeatedly
to the limit of work hardening at ordinary temperature or a
cryogenic temperature with liquid nitrogen. By use of the die 2,
external force are applied to the raw shape memory alloy 1 from
every direction, and thereby most of the alloy crystals which have
been produced upon the solidification of the ingot of the alloy or
subsequent hot working and which are random in sizes and
orientations are broken. However, even the raw shape memory alloy 1
is worked as such, since there is a degree of freedom in the
tensile direction, a martensite-like component which causes
contraction remains in the structure of the alloy. This component
has an anisotropy in the tensile direction and becomes an important
element which provides the crystals with a growth direction upon
recrystallization in the step 2 which is explained hereunder. It is
considered that such state of the raw shape memory alloy 1 after
the cold working is amorphous-like one where the crystals are
crushed almost completely, with the anisotropy being left in the
longitudinal direction.
Though the cold working may be performed at ordinary temperature as
stated before, it is preferable that it is performed at a cryogenic
temperature, such as that of liquid nitrogen, which is sufficiently
lower than the temperature singular point B. The purpose is to
transform non-martensite structures remaining in the alloy, even if
the amount of them are very small, to the martensite completely. In
general, the so called martensite finished point M.sub.f is the
temperature which is measured with respect to a specimen completely
annealed, and in actual worked shape memory materials there remain
a considerable amount of non-martensite structures even at that
temperature. The non-martensite structures may be retained
austenite, structure resulted from work hardening or the like. In
this step 1 it is essential that the raw shape memory alloy 1 is
worked so that the non-martensite structures remain as little as
possible. If the austenite or the like component remains, in
certain conditions of the worked alloy, sometimes it makes it
possible for reversible slips to occur in the alloy, even if they
are partial, and disturbs recrystallization with an anisotropy, and
consequently making the following processes incomplete. This may
eventually exerts a bad influence on the service life of the shape
memory alloy with regard to the shape recovery rate and elongation
thereof. Care should be also taken to a temperature rise due to
work heat of the die 2. Particularly in case of Ti--Ni and
Ti--Ni--Cu based shape memory alloys, the deformation resistance
has a tendency to largely depend on the strain rate and thereby
heat generation is easy to occur. With great stresses and a
temperature rise, since the martensite and the austenite are
present in a mixture, the martensite which is weaker in strength
than the austenite is broken with priority and the austenite is
liable to remain. It is difficult for the austenite which has
completely transformed to have a directionality, and thereby an
anisotropy in the tensile direction cannot be obtained. Therefore,
care should be taken to the high speed work. Severe cold working at
a temperature which is sufficiently lower than the temperature
singular point B, such as that of liquid nitrogen, can realize the
state which is almost ideal in this step. Under such a temperature,
since almost the entire of the austenite in the raw shape memory
alloy 1 is transformed to the martensite, the entire of the
structure of the raw alloy 1 is broken uniformly except the
martensite having the orientation suitable for the tensile
direction. Stresses exerted by the remaining martensite become a
factor presiding over the anisotropy of the recrystallization in
the step 2 which will be explained hereunder.
By the way, besides wire drawing, cold rolling and shot blasting
are effectual as the severe working. If the raw shape memory alloy
is manufactured by sputtering or plating, it is thought that the
structure thereof is already in an amorphous-like state, and
thereby it is not necessary to break the crystal structure thereof
by the severe cold working as in the step 1.
(Step 2)
The raw shape memory alloy 1 which has undergone the step 1 is
fixed to a restraining device 3 at the both ends thereof, as shown
in FIG. 5, with appropriate tension applied thereto. Consequently,
the raw shape memory alloy 1 is subject to a stress in the tensile
direction with the strain thereof restrained. Under such condition
the raw shape memory alloy 1 is heated for a few seconds to several
minutes to the temperature at which the recrystallization begins or
a little above. By this, a substantially uniformly fine-grained
equiaxed crystal structure with an anisotropy in the tensile
direction is produced. The reason is that, it is thought, a large
internal tensile stress is caused by heating due to the anisotropy
in the tensile direction, and the recrystallization advances
preferentially in such a direction that the internal stress is
gradually relieved. When the raw alloy 1 is processed into such a
state, the final size stability and movement property of the shape
memory alloy is improved. There is not a severe restriction as to
the magnitude of the stress applied to the raw shape memory alloy 1
prior to heating and restraining, because similar effects can be
expected in a wide range thereof A deformed component which can be
restored upon heating remains to some extent in the raw shape
memory alloy 1 which has been subject to the severe cold working as
in the step 1. Therefore, in this step, even if the alloy 1 is not
subject to a stress and just restrained in its length so as not to
become loose in the absence of a load, it attempts to contract upon
heating, thereby a stress being produced therein, and consequently
almost the same result can be attained as when the alloy is subject
to a stress and the strain produced therein is restrained as stated
above. Accordingly, such a condition can be employed as well. On
the other hand, when the raw shape memory alloy 1 is restrained
with a high level of stress applied thereto, the excessive stress
is relaxed during the recrystallization and thereby it has little
effect, but the finished shape memory alloy is deteriorated in the
size accuracy. For instance, when the alloy in the shape of a wire
is subject to an excessive tensile stress, it becomes thin.
Basically, it is enough if the raw shape memory alloy 1 is loaded
with an adequate stress in the tensile direction when the recovery
recrystallization begins. What is essential is that the raw alloy 1
is subject to as little stress or constrain as possible except
those in the tensile direction during the recrystallization.
Actually in this embodiment, the raw alloy 1 is constrained with a
stress of 10 to 100 Mpa applied thereto.
By the way, when mass production of the shape memory alloy in
accordance with the present invention is considered, using a tunnel
kiln, a similar process can be achieved, performing a similar
heating treatment, while the raw shape memory alloy is subject to a
stress by keeping an external force acting thereon instead of
restraining it as stated above. However, in that case, perhaps
because the obtained crystal structure which is fine-grained with
the crystal orientations arranged is partly destroyed, a finished
shape memory alloy is not so excellent in it properties as in the
case where the raw alloy is restrained, and the control of the
stress is difficult.
It is thought that the effect of the restraint with the stress
applied to the raw alloy is as follows. In the material which has
undergone the step 1 the formation of crystals due to the
recrystallization is caused with priority in a part where a greater
deformation is imparted such that the lattice structure is more
disturbed and the stress field becomes stronger. When the crystal
formation is achieved with the stress due to the external force in
the tensile direction applied to the alloy, both the interior of
the crystal grains and the grain boundaries come to a state where
residual stresses and strains are eliminated in equilibrium with
the stress. When from the raw alloy 1 thus processed the stress is
removed by removal of the external force or constraint after
cooling, the equilibrium of the internal stress which has been
relaxed is disturbed and the raw alloy 1 becomes a material which
structurally has a residual stress field being directional in the
tensile direction therein. Besides, it is thought that generally
when a crystal is formed, the impurity concentration is far richer
outside the crystal being formed than the inside thereof and at
last the impurities concentrate at the grain boundary
(constitutional supercooling phenomenon). The impurities may be
substances such as carbon, carbide, oxide and the like which differ
in composition from the most part of the raw alloy 1. By means of
the step 2, the impurities settle at positions where they are
stable under the stress, and after cooling, with the stress
removed, they are located partially in the tensile direction. It is
thought that such anisotropy of the recrystallization and
partiality in the tensile direction due to the impurities
constitute an elastic energy barrier which prevents a plastic
deformation from occurring and a cause of a stress field which
induce the two-way shape memory effect. Moreover, the anisotropy
facilitates the next step 3 and subsequent steps. As a matter of
fact easiness of the two-way shape memory effect appearance depends
on the carbon concentration.
In the step 2, the stronger covalent bonding property of the alloy
is, the easier it is to produce fine crystal grains therein,
perhaps because the less the thermal conductivity of the alloy is.
At present it is easier to produce fine crystal grains in
Ti--Ni--Cu based alloys than in Ti--Ni based ones. Though it is
strictly a matter of comparison, when the heating temperature is
too high or the heating time is too long, the finished shape memory
alloy is inferior in properties as an actuator and unstable as a
material, perhaps because the structure at around grain boundaries
are lost or the crystal grains become too large. In general, there
is a tendency that the larger the crystal grain sizes of shape
memory alloys are, the larger the shape recovery strain and the
shape recovery force are. However, in this treatment method, a good
result is obtained when the raw alloy crystal structure is made as
uniformly fine-grained and equiaxial as possible, having the grain
size of a several microns or less, which grain size is small for
ordinary metal materials. The reason for this is that the
subsequent process of arranging crystal orientation is thought to
be more important and the crystal grains are easy to rotate when
their sizes are small and substantially uniform. Besides, it is
thought that there is a grain size which is suitable for the
repetition movement of the shape memory alloy and stable and it
seems to be comparatively small. The optimum grain size for the
treatment in accordance with the present invention also depends on
material, shape and size of the raw alloy.
(Step 3)
After the completion of the step 2, as shown in FIG. 6, the raw
alloy 1 is newly subject to a large tensile force F.sub.1 under a
free tensile condition without constraint with regard to the
cross-sectional direction at a cryogenic temperature which is
sufficiently lower than the temperature singular point B and at
which it is completely in martensite state, until the reaction
force increases rapidly, and a deformation is imparted thereto in
the tensile direction. Since sometimes the temperature singular
point B is changed by a great stress and deformation, the above
described cryogenic temperature is obtained using dry ice or liquid
nitrogen. As such, it is thought that both the interior of the
crystal grains and the grain boundaries are completely in the
martensite state. The principal point is that the raw alloy 1 is
deformed in a state where neither within the crystal grains nor the
grain boundaries the austenite phase remain. Especially the
interior of the crystal grain, being very soft, is readily deformed
by the external force and does not resist it in the range where the
reversible slip of the atoms occur as described before. This huge
deformation strain within the crystal grain reaches to tens to
hundreds times the elastic strain seen with common metals. On the
other hand, the structure at and around the grain boundaries which
is situated between crystal grains having different orientations
and is restrained by them cannot move freely, unlike the structure
within the crystal grains, and consequently, with deformations of
the neighboring crystal grains, is deformed particularly in a
direction wherein the crystal grains slide against each other in
accordance with the external force. This huge slip deformation is,
for the structure at and around grain boundaries, a plastic
deformation which exceeds the reversible slip range. In the alloy 1
as a whole the external force is relieved and a deformation is
produced in such a way that the strain are stored at the structure
at and around the grain boundaries. During this process it is
necessary not to apply the force to the raw alloy 1 so excessively
that the plastic deformation reaches to the interior of crystal
grains. The limit of the force is easily learned by observing
consecutively a stress-strain diagram as shown in FIG. 7. In the
case that the raw shape memory alloy 1 is in the shape of a wire as
in this embodiment, when it undergoes a free tensile deformation
without external forces other than that in the tensile direction
applied thereto at a cryogenic temperature, the deformation occurs
with a comparatively small force to a certain point, but then
abruptly the reaction force increases, and so the stress. The limit
of the force is learned from the point at which the stress
increases abruptly. In the event that an excessive deformation is
imparted to the raw alloy 1 in disregard of the magnitude of the
reaction force, the plastic deformation reaches to the interior of
the crystal grains, causing a fear of internal defects occurring in
the alloy and its abrupt rupture. In general, it is preferable to
apply a stress of 300 to 500 Mpa to the raw alloy 1.
In order to obtain more excellent properties in the tensile
direction, preferably a free tensile deformation wherein there is
no restraint except in the specific direction, or the like, is
subject to the raw alloy 1, as in this embodiment. When an alloy
having comparatively small cross section is deformed in such state,
rotations and slips between the crystal grains occur easily,
because constraint is small in the cross section. On the contrary a
high level of deformation, such as that by wire drawing, which
restrains even movement of the crystals in the raw alloy decreases
the effect of this step.
(Step 4)
After the completion of the step 3, the raw shape memory alloy 1 is
heated to the vicinity of the temperature singular point S at a
heating rate which does not cause the deposition and diffusion (for
instance, 100 to 200.degree. C./min) with a tensile fore F.sub.2
which is smaller than that in the step 3 being applied thereto, as
shown in FIG. 8, in a free tension manner without restraint in the
cross-sectional direction thereof, and thereafter cooled. The force
F.sub.2 is selected to be such a small one that it will not cause a
deformation continuously in the tensile direction. In this step,
also, it may be better to say that the strain is not imparted
forcibly but the stress is controlled. In general, preferably the
stress is 100 to 200 Mpa. Similar result is obtained when the raw
alloy 1 is heated to the temperature singular point S under
constraint with being pre-deformed in the tensile direction, since
a shape recovery force is produced. But in this case the strain
under constraint is difficult to control. In this step the interior
of the crystal grains become the austenite phase which is hard, and
thereby the structure at and around grain boundaries is brought
into a state where it is restrained. At the temperature S, the
structure within the crital grains, having no excessive deformation
and being comparatively well-ordered in its atomic arrangement, is
stable and seldom makes a change. On the other hand, the structure
at and around the grain boundaries, where a high level of
crystalline distortions due to the large plastic deformation have
been induced in the step 3, is thought to be higher than that
within the crystal grains in the elastic energy level or the level
of mechanical energy which tries to restore the crystals to their
original state. Therefore, the structure at and around the grain
boundaries is liable to undergoes a change like the
recrystallization and revert to a more stable status by less heat
energy. Thus in this step 4 the structure at and around the crystal
grains alone selectively undergoes irreversible slip deformations
and consequently the adjoining crystal grains slide along each
other so that the tensile force from the outside is relaxed. Taking
a broader view of it, it means that, when the respective crystal
grains take place a reversible deformation due to the shape memory
effect, they rotate so that they are arranged in their orientations
and can move more smoothly. In other words, all of the crystal
grains are arranged in a direction in which the movement of the
shape memory alloy in the expected operational direction, namely
the tensile direction, is obstructed less. Since crystal grains of
shape memory alloys have many crystal planes in three dimensions,
where reversible deformations referred to as variants readily occur
(for instance, in case of a Ti--Ni based shape memory alloy, there
are as much as twenty four (24) orientations along which the
deformations referred to as variants can occur), with a
comparatively slight rotation each of the crystal grains can settle
in the direction suitable for the deformation in the tensile
direction. Once settled in the stable direction, each of the
crystal grains can take place a reversible deformation to the
maximum when the alloy as a whole is subject to a tensile
deformation. Accordingly, a force rotating them further is hardly
produced. In other words the alloy becomes stable as a material. In
the event that the step 2 is not carried out well and consequently
the crystal grains are uneven in their size, excessive stresses and
deformations are produced in the interior of crystal grains which
lacks conformity and the alloy becomes materially unstable. In case
the load, temperature and heating time are not adequate, the
crystal grains do not rotate, and moreover, the change reaches even
the interior of the crystal grains, and consequently the properties
of the alloy become deteriorated.
The phenomenon which occurs in the steps 3 and 4 which is
associated with the fine-grained polycrystalline material seems to
be that similar to the ultra fine grain super plasticity. A great
difference between the phenomenon related to the present invention
and the ultra fine grain super plasticity which heretofore has been
known is that in the present invention the process is finished
before the stage where a continuous deformation lasts is reached.
However, when the alloy is held for a longer time at a heating
temperature higher than the singular point S and deformed slowly,
sometimes a large permanent strain is produced.
(Step 5)
If necessary, the step 3 is carried out again with the raw shape
memory alloy 1 which has undergone the step 4. Generally, there is
a tendency that, when the process of the steps 3 and 4 is carried
out once, most of the crystal grains are successfully arranged in a
direction suitable for the expected operational direction, and even
if the process is repeated, the effect is decreased logarithmically
with the number of repetitions. However, the result of the steps 3
and 4 differs with alloys and in some cases the number of
repetitions delicately affects properties of the finished shape
memory alloy. Therefore, in some cases, as the steps 3 and 4 are
repeated alternately, the properties of finished shape memory alloy
are improved gradually. The reason for this is thought to be that
in certain cases the intermetallic compound which forms the alloy
has a smaller number of orientations in which variants are easily
produced, depending on impurities included therein and the
composition and histories thereof. In practice it is preferable to
determine the number of repetitions from results of a operation
test for the shape memory alloy with which all the processes of the
treatment have been completed once. One standard judgement to
determine the appropriate number of repetitions is to confirm that
the stress when the alloy undergoes a deformation at the cryogenic
temperature becomes sufficiently smaller than that in the first
step 3 or zero.
(Step 6)
The raw shape memory alloy 1 is repeatedly heated and cooled
between a maximum heating temperature and a minimum cooling
temperature with a force applied thereto. The maximum heating
temperature is selected to be in the vicinity of the temperature D,
and the minimum cooling temperature is selected to be the M.sub.f
point or below, preferably a cryogenic temperature similar to that
in the step 3. The force is selected to be larger than that which
is expected to be applied to the shape memory alloy when it is used
as an actuator but not so large as to damage it. Though it depends
on circumstances, in general a stress of 100 to 300 Mpa is thought
to be preferable. In this step the movement of the alloy 1 by the
heating and cooling cycle should not be restrained. It is more
effective to set the magnitude of the force to be larger upon
cooling than upon heating. This step work hardens the structure at
and around the grain boundaries adequately to secure the
dimensional stability of the alloy and induces an elastic energy
field in the alloy in a direction opposite to that of the shape
recovery of the alloy due to the shape memory effect, as is the
case with conventional training processes of shape memory alloys.
The completion of this step finishes all the processes of the
treatment.
The curve I in FIG. 9 shows an example of a temperature-strain
characteristic of a Ti--Ni--Cu based shape memory alloy obtained by
this embodiment. In FIG. 9 characteristics of conventional shape
memory alloys for actuators (curves II and III) are also shown for
comparison. FIG. 10 shows test conditions for measuring the
characteristic of FIG. 9, wherein relations between the temperature
and shrinkage displacement (contraction strain .epsilon.) of the
respective shape memory alloys 1' in the shape of a wire are
measured in a thermostat (constant temperature oven) controlled at
the temperature change 10.degree. C./min with a load of 100 Mpa to
the shape memory alloys 1'. As shown by the curve I in FIG. 9, as
for the shape memory alloy obtained by this embodiment the
temperature hysteresis is almost zero in a comparatively wide
range. Both the conventional shape memory alloy shown by the curve
II, which is of a high temperature type that operates at a
comparatively high temperature, and the conventional shape memory
alloy shown by the curve III, which has been processed with the
medium treatment, exhibit large hysteretic characteristics.
FIGS. 11 through 16 show a second embodiment of the method of
treating a shape memory alloy in accordance with the present
invention. In this embodiment it is expected that the finished
shape memory alloy takes the shape of a coil or helical spring, and
when used as an actuator, it contracts to the memorized (original)
coil length upon heating, while it relaxes and elongates to the
original deformed coil length at a low temperature upon cooling
(namely it operates as an extension spring), or it elongates to the
memorized coil length upon heating, while it relaxes and contracts
to the original deformed coil length at a low temperature upon
cooling (namely it operates as a compression spring). In this
embodiment the expected operational direction is a twisting
direction.
(Preparatory operation)
An operation similar to the preparatory operation in the first
embodiment is carried out.
(Step 1)
An operation similar to the step 1 in the first embodiment is
carried out to prepare a raw shape memory alloy 1 in the shape of a
wire having predetermined diameter. Though an anisotropy in the
tensile direction remains in the raw shape memory alloy 1, it has
substantially no effect on the characteristics of the finished
shape memory alloy to be obtained at the end.
(Step 2)
The raw shape memory alloy 1 which has undergone the step 1 is
twisted sufficiently in the expected operational direction as shown
in FIG. 11 to receive a twisting deformation, and then, restrained,
as it is, by a constraining device 3 as shown in FIG. 12. Though
the twisting deformation may be achieved at ordinary temperature,
it is preferable that it is performed at a cryogenic temperature
which is sufficiently lower than the temperature singular point B
for the same reason as in the first embodiment. Thereafter the raw
shape memory alloy 1 is heated for a short period of time to the
temperature at which the recrystallization begins or a little above
while constrained as stated above. Then a great internal shearing
stress is produced in the alloy 1 due to its anisotropy in the
twisting direction, and the recrystallization occurs preferentially
in such a direction that the internal shearing stress is relieved,
and consequently a substantially uniformly fine-grained crystal
structure having an anisotropy in the twisting direction is
produced.
(Step 3)
The raw shape memory alloy 1 which has undergone the step 2 is
subject to an additional twisting deformation in the same direction
with a large twisting force as shown in FIG. 13 at a low or
cryogenic temperature at which it is completely in martensite state
until the reaction force increases rapidly. Hereupon the twisting
torque imparted to the raw alloy 1 should be controlled so as to
prevent the plastic deformation from reaching to the interior of
crystal grains as in the first embodiment. The deformation should
be restrained as little as possible except in the twisting
direction.
(Step 4)
As shown in 14, the raw shape memory alloy 1 which has undergone
the step 3 is wound around a core bar 4 having a round
cross-sectional shape so that the twisting deformation may not be
dissolved. The raw alloy 1 may be wound while being twisted. In the
drawings, a part where one end of the raw alloy 1 is fixed to the
round core bar 4 is denoted at 5". Whether the finished shape
memory alloy 1 is to form an extension spring or compression
springs depends on the winding direction. FIG. 14 shows the case
where the finished shape memory alloy is to form an extension
spring. In the case that the finished shape memory alloy is to form
an compression spring, the raw alloy 1 is wound around the core bar
4 in the opposite direction. When the finished shape memory alloy
is supposed to form an extension spring, if the raw alloy 1 is
wound around the core bar 4 while strongly twisted, it forms a coil
shape for itself rather than being forcibly wound around the core
bar 4.
(Step 5)
Next, while being restrained in the state where it is wound around
the core bar 4 and twisted as shown in FIG. 15, the raw alloy 1 is
heated to the temperature singular point S at a heating rate which
does not cause the deposition and diffusion (for instance, 100 to
200.degree. C./min) and thereafter cooled. Consequently, the
crystals of the raw alloy 1 is reoriented along a direction
suitable for the expected operational direction, namely the
twisting direction, as is the case with the first embodiment. Since
in the step 4 the raw alloy 1 is subject to a bending deformation
as well as the twisting deformation, a higher level of deformation
may be imparted to it, as compared with the first embodiment,
inducing work hardening in some parts of it. Therefore, there are
cases where it is preferable to determine the heating temperature
to be little higher and the heating time to be short in order to
remove excessive work hardening.
(Step 6)
The core bar 4 is pulled out from the raw alloy 1, and at a
cryogenic temperature the coil of the raw alloy 1 is deformed so as
to be elongated as shown in FIG. 16 when it is of a extension type,
while it is deformed so as to be compressed when it is of a
compression type. Instead of it, mere cooling the raw alloy 1 to a
cryogenic temperature while it is still wound around the core bar 4
is also effective to the some extent, perhaps because a stress
remains in the raw alloy 1. There are cases where it improves
further the performances of the finished shape memory alloy to
stretch properly the coil of the raw shape memory alloy 1 which has
been obtained as stated above and thereafter to repeat the steps 3
to 6 several times.
(Step 7)
When necessary, the raw shape memory alloy 1 obtained by the step 6
is subject to a heat cycles of more than a few cycles between a low
or cryogenic temperature and the temperature D while the raw shape
memory alloy 1 is subject to a force in the expected operational
direction without constraining the deformation thereof. This step
is a running-in or training process which corresponds to the step 6
in the first embodiment. Upon completion of this step, all
processes of the treatment is completed.
The present invention can be applied to shape memory alloys which
are different in their shapes and movements from those in the above
embodiments. Even if manners of deformation are different, basic
processes of the treatment are same.
Although preferred embodiments of the present invention have been
shown and described herein, it should be apparent that the present
disclosure is made by way of example only and that variations
thereto are possible within the scope of the disclosure without
departing from the subject matter coming within the scope of the
following claims and a reasonable equivalency thereof.
* * * * *